Metal-oxide-semiconductor field-effect transistors (MOSFETs) are widely used in the electronics industry for switching, amplification, filtering, and other tasks related to both analog and digital electrical signals. Conventional planar MOS transistors include a gate dielectric overlying a channel region at the upper surface of a silicon substrate and a gate electrode situated above the gate dielectric. Source and drain regions are formed in the substrate on either lateral side of the channel. In operation, the gate electrode is energized to create an electric field in the channel region of the substrate, thus inverting a thin portion of the channel conductive underneath the gate dielectric and allowing minority carriers to travel through the channel between the source/drains. The threshold voltage (Vt) of a transistor is the gate voltage value required to render the channel conductive by formation of an inversion layer (e.g., in which the concentration of minority carriers exceeds that of majority carriers) at the surface of the semiconductor channel under the gate.
Scaling is a continuing process in the manufacture and design of semiconductor products, wherein electrical device feature sizes are being reduced to increase device density, improve performance (e.g., increase switching speed), and to reduce power consumption. For instance, it is desirable to scale or reduce the length of the transistor gate and hence the length of the channel between the source/drains, to increase drive current performance, particularly for operation with reduced gate voltages. The length of the gate structure is typically the smallest dimension in a planar transistor. However, lithography generally limits the extent to which transistor dimensions can be reliably scaled, wherein the minimum gate length is typically limited to the smallest dimension that can be reliably and repeatably patterned and etched using current photolithographic and etching techniques.
In addition to fabrication process limitations, performance limitations are also a barrier to scaling conventional planar transistor dimensions, particularly the gate length. For example, as the gate length is reduced, the transistor performance may be inhibited by short channel effects. In devices having long channel lengths, the gate voltage and the resulting field primarily control the depletion of charge under the gate. In shorter channel devices, however, the channel region is also affected by the source and drain voltages, leading to increased off-state current due to Vt roll off, degraded subthreshold slope, and degraded output current. In addition, since less gate voltage is needed to deplete the shortened channel, the barrier for electron injection from the source to the drain decreases, a situation sometimes referred to as drain induced barrier lowering (DIBL).
As the performance and process limitations on scaling planar transistors are reached, attention has been recently directed to transistor designs having multiple gates (e.g., non-planar MOS transistors). In theory, these designs provide more control over a scaled channel by situating the gate around two or more sides of the channel silicon, wherein a shorter channel length can be achieved for the same gate dielectric thickness or similar channel lengths can be used with thicker gate dielectrics. FIGS. 11a and 11B illustrate examples of some multiple-gate transistor designs, including dual and triple-gate transistors 60 and 62, respectively in FIG. 11A, as well as a quad-gate transistor 64, and a “PI”-gate transistor 66 in FIG. 11B, formed in a silicon over insulator (SOI) wafer 68. In conventional multi-gate devices, an SOI wafer is provided, which includes a substrate with an overlying oxide insulator and a 20.0-50.0 nm thick semiconductor layer above the oxide. The upper silicon layer is etched away, leaving isolated islands or blocks of silicon, and a gate is formed around the silicon blocks, with the ends of the blocks being doped to form source/drains, as illustrated in FIGS. 11A and 11B.
Multi-gate designs offer the prospect of improved transistor performance by alleviating the short channel effects seen in scaled planar transistors. This is due primarily to the ability to invert a larger portion of the channel silicon because the gate extends on more than one peripheral side of the channel. In practice, however, the conventional multi-gate approaches have suffered from cost and performance shortcomings, because SOI wafers are more expensive than ordinary silicon substrates and because the channel surface has been etched while carving the upper SOI silicon layer into islands or blocks. Accordingly, there remains a need for improved transistor devices and manufacturing techniques to realize the advantages of scaling while mitigating or avoiding short channel effects and the shortcomings of traditional multi-gate transistors.